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Re: Re: while modifier

by dragonchild (Archbishop)
on Sep 21, 2001 at 01:21 UTC ( #113710=note: print w/replies, xml ) Need Help??


in reply to Re: while modifier
in thread while modifier

Another way to look at it would be that $foo_text is being re-declared every iteration. The declaration is scoped within the while loop. Yes, it is a statement modifier, but it's also a looping construct with its own scope. For example:
my $x = 0; do { my $y = $x + 3; print "$x => $y\n"; $x++; } while ($x < 10);
A little explanation - do (essentially) says to treat the following block as a single statement. Thus, you can use statement modifiers like post-whiles and post-foreachs. Now, $y is very obviously scoped within the confines of the while-loop. It's the same concept for while-modifiers.

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Re:while modifier( and scoping)
by demerphq (Chancellor) on Sep 22, 2001 at 20:19 UTC
    Hi Dragonchild, actually this isn't quite right

    Yes, it is a statement modifier, but it's also a looping construct with its own scope.

    In fact modifiers of all kinds do not have their own scope. They execute in the scope of their enclosing block. This can be seen by my $x=1 if $y;$x++; where usage of the the $x after the if does not raise an exception.

    The reason it seems like they do when you use the do BLOCK pseudo-function is because in this case the block itself has its own scope. This is illustrated by the code below where the only variable usage that throws use strict; is the one declared inside of a do block.

    Another interesting point with the do usage is that the block gets executed an extra time, ie before the first evaluation of the conditional. This can be seen by the output line Do's pre-evaluation:DO{}while

    The compound statement forms of the modifiers are different in that they have their own scopes and scoping rules (for instance the for my $i (0..10) {} where the my's scope is actually the contents of the block.

    use strict; use warnings; use Tie::Cycle; tie my $cycle, 'Tie::Cycle', [ 1..3,0,reverse (1..3),0 ]; $\=":stmt while\n"; print my $statement=$_ while ($_=$cycle)>0; print defined($statement) ? "Defined" : "Notdefined"; $_="Do's pre-evaluation"; $\=":do{}while\n"; do { print my $do = $_ } while ($_=$cycle)>0; eval 'print defined($do) ? "Defined" : "Notdefined";'; print $@ if $@; $\=":stmt while 0\n"; print my $while="Test" while 0; print defined($while) ? "Defined" : "Notdefined"; $\=":stmt if\n"; print my $if="Test" if 0; print defined($if) ? "Defined" : "Notdefined"; $\=":stmt for\n"; print my $for=$_ for 0; print defined($for) ? "Defined" : "Notdefined"; __DATA__ 1:stmt while 2:stmt while 3:stmt while Notdefined:stmt while Do's pre-evaluation:do{}while 3:do{}while 2:do{}while 1:do{}while Global symbol "$do" requires explicit package name at (eval 1) line 1. :do{}while Notdefined:stmt while 0 Notdefined:stmt if 0:stmt for Notdefined:stmt for
    Notice that in the non do {} simple statement form, even when the condition is 0 and one might expect Perl to optimize it away the variable is created in the outside scope.

    Yves
    --
    You are not ready to use symrefs unless you already know why they are bad. -- tadmc (CLPM)

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